Electric induction furnace



Sept. 27, 1932. F. LlNNHoFF 1,879,361

ELECTRIC INDUCTION FURNACE Filed July 22. 192g Inventor'.- Franz Linrzzofff Attorney Patented Sept. 27, 1932 UNITED STATES PATENT OFFICEFRANZ LINNHOFF, F EBERSWALDE, GERMANY, ASSIGNOR TO AJAX ELECTROTHERMICCORPORATION, 0F AJAX PARK, NEW JERSEY, A CORPORATION 0F NEW JERSEYELECTRIC INDUCTION FURNACE Application led J'uly 22, 1929, Serial No.380,140, and in Germany July 24, 1928.

My invention rela-tes to electric induction furnaces, more particularlyto coreless induction furnaces, in which the melting material issurrounded by an induction coil and heated 1by the eddy currents inducedin it by the It is known to decrease the leakage of the induction coilby surrounding it with a yoke by which the magnetic circuit is closed.The

yoke must consist of laminated iron to avoid high losses caused by theeddy currents induced in it. Such yokes have been made in form ofseveral columns arranged round about the coil, the single columns havingbeen connected one to another at the top and at the bottom. It would bemuch more advantageous to provide a yoke of hollow cylindrical shape,but the production of such yokes of laminated iron causes very highcosts and gives great constructive diiculties. This is ,the case evenwith the known columnated yokes of laminated iron.

The object of my invention is to provide a yoke which does not cause anyeddy current losses and can be made in any form without difficulties.

My invention consists in a yoke of pressed powder with little magneticand high electric resistance. Such powder, which may consist f. i. in amixture of iron parts with zinc oxide or chromium oxide, with or withoutapplication of a binding substance, can be pressed into any convenientform with little cost and is adapted for magnetic yokes even better thanlaminated iron since its electric resistance is higher. The powder caneasily be pressed into a hollow cylindrical form so that using such ayoke of magnetic powder the best magnetic conditions can be attained.Moreover, the yoke of magnetic powder makes it possible, owing to itshigh electric resistance, to imbed the windings of the induction coilimmediately into the yoke without any insulation. Thereby the supportingdevices for the coil become available and the side-leakage of the coilis entirely prevented.

In the accompanying drawing there is illustrated one embodiment of theinvention.

Figure 1 is a vertical central section of a coreless induction furnaceembodying my invention.

Figure la is a fragmentary vertical section showing a modified form.

Figure 2 is a top plan view of the structure seen in Figure 1.

The Crucible 1 of an induction furnace is surrounded by an inductioncoil 2 and provided with a cover 3. The coil2 is surrounded by amagnetic yoke 4 of magnetic powder pressed into a hollow cylindricalform The magnetic. powder preferred by me consists of a mixture of ironparts and zinc oxide or chrome oxide with or without a binder vhichcheaply can be pressed into convenient orm.

The furnace is supported by a frame 5 and can be poured out through thespout 6 by tilting itover the trunnions 7.

In the modification shown in Fi re 1a the magnetic powder has beeninsert d between and about the individual turns of the induction coil.

Having now particularly described and ascertained the nature of myinvention and in what manner the same is to be performed, what I claimis:

1. In an electric induction furnace, in combination, a crucible, aninduction coil surrounding said crucible and a yoke of pressed magneticpowder arrangedaround said coil.

2. In an electric induction furnace, in combination, a crucible, aninduction coil surrounding said CrucibleV and a yoke of pressed magneticpowder arranged around said coil, said powder consisting 1n a mixture ofiron parts and zinc oxide.

3. In an electric induction furnace, in combination, a crucible, aninduction coil surrounding said crucible and a, yoke of pressed ma neticpowder arranged around said coil, sai powder consisting 1n a mixture ofiron parts and chrom oxide.

4. In an electric induction furnace, in combination, a crucible, aninduction coil surrounding said crucible and a hollow cylindrical yokeof pressed magnetic material surrounding said coil.

5. In an electric induction furnace, in combination, a crucible, aninduction coil surrounding said crucible and a yoke of magnetic powderforming a return magnetic circuit for the induction coil havingsubstantially continuous poor electrical conductivity and good magneticpermeability in a direction parallel to the axis of the coil.

6. In an electric induction furnace, in combination, a crucible, aninduction coil surrounding said Crucible and a yoke of pressed lomagnetic powder arranged around said coil, said powder consisting in amixture of iron parts and zinc oxide.

7. In an electric induction furnace, in combination, a crucible, aninduction coil surrounding said Crucible and a yoke of magnetic powderwith high electric. resistance, the windings of said induction coilbeing embedded into said yoke.

8. The method of facilitating magnetic return about a furnace inductorwhich consists in reducing the reluctance of the magnetic path betweenindividual turns of the inductor and concurrently reducing thereluctance of the return path outside of the turns of the inductor.

9. In an electric induction furnace, a crucible, an inductor coilsurrounding the crucible and an annular powdered magnetic path of highelectrical resistance surroundsu ing the inductor coil.

10. In an electric induction furnace, a crucible, an inductor coilsurrounding the crucible and powdered magnetic material against theoutside of the inductor coil throughout its length.

11. In an electric induction furnace, a crucible, an inductor coilsurrounding the Crucible and a. magnetic return circuit of highresistance for the flux induced by the coil 0 including powderedmagnetic material of high electrical resistance between the'turns of theinductor coil.

12. In an electric induction furnace, a crucible, an inductor coilsurrounding the crucible and magnetic material of high electricalresistance about the inductor coil and between its turns near theiroutside edges.

In testimony whereof I affix my signature.

FRANZ LINNHOFF.

